A digital-to-analog converter (DAC) is a device or circuit element that converts digital codes into a continuous range of analog signal levels, i.e., the device constructs an analog signal from a digital signal. Resolution, sampling rate, speed, and linearity determine the quality of performance of a DAC. Higher resolution and greater linearity with increasing speed are preferred.
One type of DAC is a binary-weighted DAC that converts a digital signal to an analog signal by weighting a voltage or a current according to each bit in the digital signal or digital code. The weighted voltage or current assigned to each bit in the digital signal or digital code are then summed together to create an analog signal. Another type of DAC is a thermometer-coded DAC design that converts the incoming binary digital signals to thermometer-coded signals that have one bit for each possible value of the incoming digital signal. The number of asserted bits in the thermometer code is proportional to the value of the incoming digital signal. When a bit is asserted, a corresponding element generates an output analog signal.
Many modern DACs are hybrid DACs that reduce die area by splitting the digital signal into portions that are then converted by sub-DACs of varying types, such as the binary and thermometer-coded DACs described above. Unfortunately, these modern conversion methods require large circuit area to implement the conversion process. For example, a modern 12-bit current steering hybrid DAC will require the use of 4096 CMOS devices to implement the digital-to-analog conversion process. Similarly, a modern 14-bit current steering hybrid DAC will require approximately 16000 CMOS devices to implement the digital-to-analog conversion process.
It is also necessary to calibrate modern DACs to decrease linearity errors caused by the fabrication process used to create the DAC. One prior art method to calibrate a DAC utilizes a higher resolution analog to digital converter (ADC) to perform the calibration during the conversion process. The increased complexity necessary to implement this calibration process can ultimately amplify linearity errors and increase the total die area needed to perform the digital-to-analog conversion. Another prior art method uses a floating gate calibration method to calibrate the DAC. However, the effects of channel charge injection and leakage current require the recharging and recalibration of the DAC, slowing the DAC down. Ultimately, including calibration processes further increase the DAC demand for circuit area.
Due to the requirements of even greater circuit area to perform a calibrated conversion process, some DACs perform the digital-to-analog conversion process without calibrating the current or voltage values within the DAC. Failure to calibrate increases linearity errors and limits the resolution of a DAC. Thus, calibration of the DAC, regardless of type, is necessary to reduce processing variations and component value mismatching and, consequently, linearity errors. Therefore, there is a need for a system and method of calibrating a DAC while also reducing the circuit area necessary to implement the digital-to-analog conversion process.